It should self-extract. You undo the sleeve nut by that central hexagon there, it comes loose on the magneto armature. You continue to undo it and a left hand thread engages in the middle of the thing and off it comes as one unit.
Very simple if it's as it should be (hope it is) and no special tool needed.
However, if it won't come it's because the self-extract system has died. Try pulling as hard as you reasonably can on the hexagon at the same time as you turn it anticlockwise (suggest with self-locking grips) - that can sometimes get a dodgy self-extract to engage. If it does, keep turning, and it should come off. If it still won't come, it becomes a pain.
These fibre pinions are fragile. Levering and hitting anything that's not steel are not recommended. You cannot dismantle the auto advance unit while it's in place, so that's a complete no-no.
Your best bet, if you're in this position, is to undo the 3 magneto securing nuts, screw the sleeve nut a few turns onto the armature thread but not all the way, and while supporting the magneto body to absorb shocks etc with one hand, tap the hex on the sleeve nut. To get a bit more free play for the sleeve nut, you can take out the horseshoe washer under the hex head (it comes), and the pegged washer under that - that gives quite a bit of extra movement. In principle a firm tap on the hex ought to push the mag off the pinion. The force of the tap is being taken by the inside of the timing cover, however, so caution is critical. If it works, then the pinion will be loose and the mag will be wobbling on its studs.
If that doesn't work, then you're in trouble. The best answer I can think of in such a case is to drill the hexagon through (the sleeve nut is hollow down below, obviously), and either -
1. tap it say 1/4 BSF or whatever moderate size, then screw in long bolt (or threaded rod with 2 nuts locked together to make a head) and use it to push the armature shaft off the taper. Or 2. you could use a hammer and drift through the hole, which is easier but cruder.
You might even be able to get an extractor with fine legs to engage under the metal arms of the auto-advance, with its central screw pressing on the rod and do it that way - gentler but finding an extractor to fit might be hard.
Fingers crossed none of this is necessary and the thing is in fine working order.
The only other thing I'd say is look very closely at the pinion's teeth when you have it in your hand. Any sign of wear at all, best replace it. Compare the profile of the teeth with the teeth on the camshaft pinion next to it. And if there is appreciable wobble of the pinion on the shaft that goes through it, then you need to think about a replacement unit. Wear sufficient to cause the thing to rotate out of perfect alignment will hasten the day the thing dies. And if it dies it will somewhere very inconvenient. Just been there and got the T shirt!
